There's a report here in which a prepaid GPRS data offer is discussed from Safaricom. Here's an excerpt:

I spent 2 weeks traveling around Kenya in October [2008?] and used my trusty old Nokia E61 to check my emails. I would then spend an hour or two in the evenings writing emails on the phone to my business partners and customers. When I first arrived in Nairobi I went to the nearest Safaricom shop and bought a pre-paid SIM card for 30 Euros. I was fully expecting to have to purchase more minutes as the two weeks progressed, however as I was not sure how the system would work I did not want to invest too much up-front.

In reality, after 2 weeks I still had 18 Euros left on the card! In Kenya you pay 8c per Mb and with the massive compression of my emails using Momail I only used 12 Euros in 2 weeks. This with VERY heavy usage. I was totally amazed. I even did some web browsing which probably ate up most of the 12 Euros..

• Data offer details here
• APN: web.safaricom.com, taken from here. Maybe there are other APN's as well, needs to be checked.